1. Field of the Invention:
This invention relates to a process for the preparation of an indigo compound. More specifically, it relates to a process for the preparation of an indigo compound by reacting an indole compound having no substituent at the 2- and 3-positions with an organic hydroperoxide in the presence of a specific additive.
2. Description of the Related Art:
Indigo compounds are important compounds that are useful as dyes. The presently employed industrial processes for the preparation of indigo comprise forming an N-phenylglycine salt from aniline and chloroacetic acid, or from aniline, cyanic acid and formaldehyde, converting this salt into an indoxyl compound by alkali fusion at elevated temperature, and then oxidizing this compound with air. However, these processes are not only complicated ones involving many steps, but also require the use of large amounts of potassium hydroxide and sodium hydroxide. Moreover the recovery and reuse of used potassium hydroxide and sodium hydroxide has the disadvantage of consuming much energy and requiring special equipment. Therefore, conversion to a simpler process has been desired.
Meanwhile, it has been reported in the field of synthetic organic chemistry that a slight amount of indigo was formed by oxidation of indole. Specifically, Obata et al. have reported that, when peracetic acid, which is a percarboxylic acid, was produced from hydrogen peroxide and acetic acid in the reaction system and reacted with indole, a trimer of indole, or 2,2-diindyl .psi.-indoxyl, was obtained and in addition a small amount of indigo was formed as a by-product (Bull. Agr. Chem. Soc. Japan, Vol. 20, pp. 80-83, 1956). Moreover, B. Witkop et al. have reported that, when perbenzoic acid, which is a percarboxylic acid, was reacted with indole in chloroform by allowing the reaction mixture to stand in a refrigerator overnight, a very small amount of indigo was formed together with a variety of other products (Justus Liebigs Annalen der Chemie, Vol. 558, pp. 91-98, 1947). Furthermore, A. K. Sheinkman et al. have reported that, when hydrogen peroxide, which is an inorganic peroxide, was reacted with indole in methanol, the trimer 2,2-diindyl-.psi.-indoxyl was obtained in high yield, as was the case with the reaction using peracetic acid, and the formation of indigo was only detected by chromatography (Khim. Geterotsikl. Soedin., Vol. 11, pp. 1490-1496, 1978). However, all of these reports were concerned with a brief investigation on the reactivity of indole, and indigo that is the desired product in the present invention was nothing but a byproduct formed in very small amounts. Accordingly, these are not satisfactory processes for the preparation of indigo compounds.